I am puzzled by the use of the accusative THN QALASSAN in Matt 14:25 and of
the genitive THS QALASSHS in Matt 14:26. It seems to me that both verses
describes Jesus' walking (movement). Is there any syntactical explanation.

I am even more puzzled by the variant readings. If you notice several
manuscripts that have the accusative in Matt 14:25 have the genitive in
14:26 and and vice versa. One would expect an harmonization of the two
mentions of Jesus' walking on the sea, i.e., two accusatives or two genitive.